The present invention relates to a method and a device for shells incorporating base-bleed for extended range and which during the first phase of their trajectory are spin stabilized but which in a subsequent phase are subjected to spin deceleration when fins that are initially retracted deploy to assume the stabilization function. The present invention involves a method and a device that enables the said shells to achieve an effective and functional fin deployment with a simultaneous ejection of the base-bleed unit at the point in time when the switch to fin stabilization is desired with regard to the function of the shell, e.g. guidance to the target. By ejecting the base-bleed unit the shell is freed of extra weight that reduces its guidability at the same time as the center of gravity of the shell is shifted forwards towards the nose section which is advantageous for fin-stabilized shells. A preferred variant of the present invention also enables the location of the driving band on the shell to be freely selected for optimal functionality which, in most cases, would probably be forward of the deployable fins.
Artillery shells are usually spin-stabilized throughout their trajectory until impact with the target or self-detonation. There are also, however, numerous types of special shells that are spin-stabilized during the firing phase in the gun barrel but whose rate of spin is subsequently decelerated at an earlier or later stage in their trajectory when the stabilising function is taken over by the fins that deploy from the shell. This applies, for example, to terminally guided shells whose projectile trajectories can be corrected by means of command activated thrusters, deployable deceleration devices or the equivalent. It is, namely, much more difficult to correct the trajectory of a purely spin-stabilized body by external means than to perform an equivalent trajectory correction for a fin-stabilized body.
A spin-stabilized shell usually displays smaller trajectory deviations than an equivalent fin-stabilized shell. Moreover, the fin-stabilized shell has a greater air resistance resulting in shorter range, and is more affected by wind conditions.
As indicated above the present invention relates to the special conditions that apply when such shells that during firing and the major part of their trajectory are fin-stabilized and which incorporate a base-bleed unit for extended range and which, furthermore, towards the end of their trajectory have their rate of spin decelerated at the same time as they become fin-stabilized when their built-in stabilizing fins are deployed.
A number of different design principles already exist for projectiles with various types of deployable fins.
In the purely theoretical type of retractable fins each fin is initially retracted radially in the projectile body or, perhaps more usually, retracted in a dedicated slot or compartment in the projectile body. For the actual deployment function in which the fins flip up or spring up radially there are usually springs incorporated for this purpose. The major disadvantages with this type of fin is that they occupy too much space in the projectile body, and that it is difficult to provide them with sufficient surface area.
A type of fin that occupies significantly less space is the type which initially, i.e. prior to deployment, is retracted snugly curved against and around the projectile body and which, after they are exposed by the ejection of a protector or the opening of a special retaining device or suchlike, flip up primarily as the result of centrifugal forces and are thereafter locked in deployed mode. An example of this type of fin, in which the fin retains its convex shape even after deployment, is Swedish patent no. 339646. However, provided one selects the correct material it is possible to manufacture fins which become virtually flat after deployment despite having been retracted against the shell body on which they are installed for a number of years. With this type of fin the surface area of the fin usually poses no problem, but on the other hand it is essential to protect the fins while in retracted mode from the gas pressure in the barrel during firing of the projectile. If the fins were subjected to the full force of the gas pressure in the barrel in most cases this would result in such deformation of the fins that their function as stabilizing devices for the shell would be at risk, not to mention whether deployment itself would still be possible after such deformation. A solution to this secondary problem is described in conjunction. with the appended example.
The combination of deployable fins and ejectable base-bleed unit is in Swedish patents Nos 7908002-4 and 8200312-0. In both these cases the fins are deployed in conjunction with ejection of the base-bleed unit, and the location of the fins forward of the shell driving band protects them from the full force of the gas pressure during firing. Although this appears to be a natural solution to the problem it can, however, not always be used as it is often other criteria that determine where the driving band shall be located along the length of the shell (projectile). Since the shell is subjected to its greatest load forces precisely at the cross-section through the driving band it is necessary to ensure that the shell is highly resistant to deformation at this cross-section. A further complication is involved if the shell is intended for dispersing bomblets/submunitions and thus must be openable, which in most cases means that the base of the shell must enable the bomblets/submunitions to be dispersed.
The purpose of the present invention is to offer a method and a device enabling simultaneous fin deployment and ejection of the base-bleed unit for shells incorporating a base-bleed unit and which are fin-stabilized in the first phase of their trajectory but which in a subsequent phase have their rate of spin retarded by fins deployed from the shell body, which fins after spin retardation take over the stabilizing function.
This complete dual function sequence as specified in the present invention is achieved by a single small gas generating charge that is ignited by a control command, which charge ejects the base-bleed unit from inside the tail unit of the shell and simultaneously ejects a fin protector that is mechanically joined to the base-bleed unit and which concentrically surrounds the shell tail unit in which the fins are mounted and against which they are convexedly retracted initially. The union between the base-bleed unit and the fin protector is the aft wall that during firing forms the base of the shell, while there is a ring-shaped space between the base-bleed unit and the fin protector which houses the actual tail unit of the shell in which the fins are mounted and retracted ready for deployment as soon as the fin protector is ejected. When the base-bleed unit is ejected out of its location in the tail unit of the shell, the fin protector is thus also removed and the fins are free to deploy.
As a rule the fins of fin-stabilized projectiles are angled a few degrees relative to the longitudinal axis of the projectile to impart an inherent low rate of spin to the fin-stabilized projectile. Such a slight angling of the fins may also be incorporated in the above indicated type where initially the fins are retracted convexedly against the projectile body, but when deployed have virtually flat surfaces. The same effect can also be achieved by slightly angling the pivot pin of each fin relative to the longitudinal axis of the projectile.
A special advantage of this method and device as described in the present invention is achieved with the version of the invention in which the space between the fin protector and base-bleed unit not occupied by the tail unit of the shell or the fins is completely filled with some sort of appropriate inert, non-combustible or non-glutinous substance with low compressibility and very low inherent strength. Then the fin protector, provided it is completely sealed while attached to the shell, can be made of a relatively thin material giving the least possible dead-weight and requiring minimum space.
For example, there are certain bi-component silicones including some sold under the name SEALGAARD, that meet these requirements. Another conceivable substance would be a suitable fluid or a thixotrope.
With the fin protector under consideration filled in this way with such a non-compressible substance that completely surrounds the retracted fins it becomes possible to use a fin protector which, although the fin protector must be fully sealed, it is perfectly adequate for it to be made of small gauge (i.e. limited wall thickness) material as it will withstand extremely high external pressure without suffering deformation that would prevent ejection when the time comes and without any deformation of the enclosed fins. This means in turn that the driving band of the shell can be located at the optimal position irrespective of whether or not the fin protector and the retracted fins inside are located on the section of the shell that is most affected by the propellant gases. Deployment of the fins then requires only ejection of the fin protector in question after which the inherent spring force of the fins when initially convex in retracted mode, centrifugal forces, and/or air resistance forces the fins outwards to their deployed mode while simultaneously slinging the low inherent strength protective substance from the shell body and fins.
Even conventional artillery shells designed for firing in field guns and howitzers can be made surprisingly thin-walled provided that the rearmost sectionxe2x80x94primarily the shell cross-section where the driving band is located-which is exposed to the greatest forces is dimensioned sufficiently robustly. For shells designed to disperse bomblets and other types of submunitions where the shell itself is merely a cargo carrier, it is especially desirable that the inherent weight of the shell body is minimal. The present invention with its ejectable base-bleed unit housed in the tail unit of the shell enables appropriate parts of the base-bleed unit to be used as reinforcement for the cross-section of the shell where the driving band is located. The weight of this reinforcement can then be removed together with the base-bleed unit when they are no longer required. An especially advantageous version is shown in the example appended in which the inner aft wall of the base-bleed unit, i.e. the wall opposite the gas outlet which must have relatively heavy-duty dimensions, is used as reinforcement of the shell cross-section where the driving band is located. Provided that the location of the driving band is otherwise optimal a weight advantage is gained in the shell after the base-bleed unit has been ejected, and it is quite irrelevant if the inner wail of the base-bleed unit must be made excessively strong for this reason since the dead-weight in the shell has been otherwise reduced.